Can a 120v signal be safely fed into an Arduino input high/low?
I'm thinking of using a thermostat at 120v as a 'sys trip' of sorts. There would be significant wattage running through the thermostat during normal operation (like 800W).
If the thermostat signal is open, then that means we hit a max temp that was too high and need to reset the machine. When the thermostat signal is again reestablished (signal high) then we can change state back to reading if the power button was pressed (to restart the machine).
I would use an AC input optocoupler with a series capacitor to limit current to the LED on the input. For the output, a capacitor in parallel to smooth the output.
Can a 120v signal be safely fed into an Arduino input high/low?
No! Lethal voltages MUST be isolated from low-voltage circuitry and especially from anything a human can touch.
There are 3 ways to electrically isolate -
Optical
Relay (electro-mechanical isolation)
Transformer (magnetic isolation AC only)
Also, the Arduino can be damaged by AC (even at low voltage) because it can be damaged by negative voltages and AC is negative half the time.
...A voltage divider could be used to knock-down the voltage and under the "right conditions" it can be safe for the Arduino, but don't be tempted! It can be dangerous to you, or to your computer if you plug-in the USB. It would be illegal to sell a product made that way unless it's completely enclosed in an insulated box with no connections other than the power connection (i.e. All of the electronics inside your DMM are insulated/isolated from the outside world so it's safe to measure AC as long as you don't touch the probes/conections).